Week-11 Challenge: Braking

WEEK 11- BRAKING

Question 1:

For a defined driving cycle, calculate the energy required for braking.

Answer:

Under the condition of braking, some amount of energy is required to do the same as that of energy required for accelerating the vehicle. Hence to calculate energy, we use the acceleration energy equarion,

Calculating the energy required for braking for a defined drive cycle:

• Drive cycle: Follow the link for Drive cycle Time vs Velocity data

https://docs.google.com/spreadsheets/d/13nHLen4RWr-UipOxB3stwWwatrf3POpL5MuRZqXd8PE/edit?usp=sharing

• Graphical Representation of drive cycle:

From the drive cycle, there are 4 different situations of decceleration of which the energy has to be calculated.

First Braking     : 24 - 15km/hr in 4 seconds

Second Braking : 35 - 30km/hr in 1 second

Third braking    : 45 - 23 km/hr in 5 seconds

Fourth braking  : 63 - 5 km/hr in 7 seconds

Using Excel Calculator:

Solution:

Total Braking energy Required = 0.114 kWh = 413KJ

Question 2:

Why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated? 

Answer:

Regenerative braking allows electric vehicles to use the motor as a generator when the brakes are applied, to pump vehicle energy from the brakes into an energy storage device. Regenerative braking is an effective approach to extend the driving range of an EV.  

• The regenerative braking torque cannot be made large enough to provide all the required braking torque of the vehicle as it might lead to the deterioration in stability of the vehicle. Also as compared to the torque values of acceleration torque of motor is much higher than decceleration torque. 
• At high speed, the motor might be in its constant power region which causes less values of torques produced and this limits the amount of braking torque that is used for regeneration. Hence the peak vlaue of accleration torque and braking torque are not equal. 

Electric and Mechanical brakes Co-ordination:

The regenerative braking system may not be used under many conditions, such as with a high State of Charge (SOC) or a high temperature of the battery. In these cases, the mechanical braking system works to cover the required total braking torque.

Thus, cooperation between the mechanical braking system and the regenerative braking system is a main part of the design of the EV braking control strategy and is known as torque blending.

There are 2 different control strategies for Braking Combinations:

1. Series Control Strategy
2. Parallel Control Strategy

Series Control Strategy:

• a combination of friction-based mechanical braking system with a regenerative braking system that transfers energy to the battery with an integrated control, which distributes the required braking force between the regenerative braking system and the mechanical braking system.
• Series Startegy could give an increase of 15-30% in fuel efficiency and has more consistent braking due to good torque blending capability.

Parallel Control Strategy: 

• combination of mechanical and the regenerative braking system, operated simultaneously without an integrated control.
• The regenerative braking force is added to the mechanical braking force which cannot be controlled.
• The regenerative torque is determined by considering the motor capacity, battery SOC, and vehicle velocity. The regenerative braking force is calculated from the brake control unit by comparing the demanded brake torque and the motor torque available.

 Application of Braking Combination : EDIB (Electric Driven Intelligent Brake) in NISSAN LEAF

•  EDIB allows and controls optimization of the regenerative brake and regular friction brake (hydraulic brake).
•  The system produces an adequate braking force keeping energy regeneration increased as much as possible.

• At higher speeds, a combination of regenerative and friction braking is applied.
• At lower speeds, regenerative braking is given a higher proportion for maximised energy conversion.

Question 4:

Make a MATLAB program which plots contour of given motor speed, torque and efficiency values. 

 Answer:

 Motor Speed - 0 - 1000rpm

Motor Torque - 0 - 700 N.m

Motor Constants:

• Copper Loss constant,    kc = 0.1
• Iron Loss Constant,        ki = 0.001
• Windage Loss Constant, kw = 0.0001
• Motor Loss Constant,     Cm = 25

MATLAB Code:

clear all
close all
clc

%Defining Speed & Torque
speed = linspace(0,1000,300);
torque = linspace(0,700,300);

%Defining Motor Constants
kc = 0.1;      % Copper Loss constant
ki = 0.001;    % Iron Loss Constant
kw = 0.0001;   % Windage Loss Constant
Cm = 25;       % Motor Loss Constant

%obtaining 2-D Grid fpr Speed & Torque
[X,Y] = meshgrid(speed,torque);

Powerout = X.*Y;                                   % Calculating output power

Totalloss = ((Y.^2).*kc)+ (ki.*X)+((X.^3).*kw)+Cm; %Total Loss Calculation

inputpower = Powerout + Totalloss;                 % Calculating input power

Z = Powerout./inputpower;  % Efficiency Calculation

%Defining Efficiency levels for contouring
EL = [0.70,0.72,0.75,0.77,0.79,0.80,0.81,0.82,0.83,0.84,0.85,0.86,0.87,0.88,0.89,0.90,0.91,0.92,0.93,0.95];
%Plotting the Efficiency wrt speed & Torque
contourf(X,Y,Z,EL)
colorbar
xlabel('Speed (rpm)');
ylabel('Torque (N.m)');
title(' Efficiency Contour');

OUTPUT:

The contour plot for a motor with defined constants are plotted in the defined Speed Vs Torque Region based on efficiency. It can be seen that for increasing speed and torque, the efficiency of the motor gradually decreases. The level of efficiency is depicted using colorbar.